Hotspot formation

Microwave-mediated reactions can also be easily carried out without solvents (see Section 4.1). The requirements for these dry media reactions are different to those for reactions in solution. As no solvent is involved, the pressure built-up is rather low, and in most instances such reactions are performed under open-vessel conditions. On the other hand, these mixtures can easily be locally overheated, even though the overall bulk temperature may be comparatively low (macroscopic hotspot formation). Stirring and accurate temperature measurement can prove rather difficult within such a matrix, impeding the investigation of certain reaction conditions. Thus, degradation or decomposition of reagents can be a severe problem for these kind of reactions. 

Name some technical measures to avoid thermal mnaway and hotspot formation. 

ATR of methane, methanol, and ethanol can be efficiently carried out in membrane-based reactors. The hotspot formation, typical of autothermal reactions, which is detrimental for the stability and selectivity of the membrane used in such reactors, can be circumvented by using FBMRs. 

Other microwave-assisted parallel processes, for example those involving solid-phase organic synthesis, are discussed in Section 7.1. In the majority of the cases described so far, domestic multimode microwave ovens were used as heating devices, without utilizing specialized reactor equipment. Since reactions in household multimode ovens are notoriously difficult to reproduce due to the lack of temperature and pressure control, pulsed irradiation, uneven electromagnetic field distribution, and the unpredictable formation of hotspots (Section 3.2), in most contemporary published methods dedicated commercially available multimode reactor systems for parallel processing are used. These multivessel rotor systems are described in detail in Section 3.4. 

Recent development of mitochondrial theory of aging is so-called reductive hotspot hypothesis. De Grey [465] proposed that the cells with suppressed oxidative phosphorylation survive by reducing dioxygen at the plasma membrane rather than at the mitochondrial inner membrane. Plasma membrane redox system is apparently an origin of the conversion of superoxide into hydroxyl and peroxyl radicals and LDL oxidation. Morre et al. [466] suggested that plasma membrane oxidoreductase links the accumulation of lesions in mitochondrial DNA to the formation of reactive oxygen species on the cell surface. 

Dietary preparation involving liquid materials frequently results in either wet feed in which the test article does not disperse or formation of gumballs feed and test material that form discernible lumps and chemical hotspots. Drying and grinding of the premix to a free-flowing form prior to mixing the final diets may be required however, these actions can affect the chemical nature of the test article. 

This approach, however, requires the absence of ill-defined carbon deposits originating from defect-induced soot formation on the surface of nanocarbons during their synthesis. Pyrolytic structures often counteract the control over activity and selectivity in catalytic applications of well-defined nanocarbons by offering an abundance of highly reactive sites, however, in maximum structural diversity. Although some nanocarbons are equipped with a superior oxidation stability over disordered carbons [25], such amorphous structures can further induce the combustion of the well-ordered sp2 domains by creating local hotspots. Thermal or mild oxidative treatment,... 

Denissenko MF, Pao A, Tang M, Pfeifer GP. (1996). Preferential formation of benzo[a]pyrene adducts at lung cancer mutational hotspots in P53. Science. 274(5286) 430-32. 

The large number of known sequences of the p53 gene from tumor patients was particularly valuable for interpretation of the crystal structure since a spectrum could be assembled for p53 mutation in association with tumor formation. The mutation spectrum shown in Fig. 14.9 shows hotspots , positions at which p53 mutations are seen particularly frequently in tumor patients. 

The formation of hotspots depends upon the energy input and the physical properties of the explosive composition. The diameter of the hotspots is in the region of 0.1-10 fim and their duration is about HTMO-3 s with temperatures greater than 900 °C. There have been various theories put forward to describe the mechanisms for the formation of hotspots, some of which are described below. 

Usually it is assumed, that on impact most primary explosives are initiated by hotspots from inter-crystalline friction. For secondary explosives, as a general rule, initiation by impact results in the formation of hotspots, which originate (adiabatic compression) from gas bubbles between the crystals. Such hotspots only exist for approximately 10 6 s. 

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